Fabrication of stencils used in thick film screening processes requires precise image generation methods to produce matching images on opposite sides of the opaque material used to form the stencils. The two image patterns must be perfectly aligned so that process chemicals acting on the opposite surfaces remove material in the pattern area in such a way to establish proper relationship of the resulting images with respect to each other. Any misalignment of the images during the stencil fabrication process results in fabrication of a defective stencil.
The current alignment process that is used by some companies in the electronic industry for alignment of images is the ball and cone method. This is a manual process where the planar master artwork plate is securely held by planar fixtures. A planar stencil or metal foil having a coating of a photoresist on both surfaces is then inserted between the two fixtures. The two fixtures holding the artwork plates are brought together and balls on one fixture are inserted into inverted hollow cones that are part of the second fixture. The fixtures provide a registration of two artwork plates to each other but there is no verification of the aligned artwork.
Similarly, the prior art does not allow for the verification of master image alignment prior to image transfer onto the stencil. The presently known techniques require that the master image plates be aligned prior to the transfer of image onto the stencil that is being fabricated. Alignment of the master image plates is assumed to be maintained during all subsequent image transfers.
Alignment before and during processing is done for a variety of reasons. For example, in a manufacturing type operation it may be important to control or continuously monitor a series of successive intervals, such as disclosed in U.S. Pat. No. 2,994,783 (Looschen) or recognition of a pattern on a workpiece to enable a subsequent operation to be executed upon the workpiece in a precise manner, such as discussed in U.S. Pat. No. 3,525,872 (Schneider).
U.S. Pat. No. 3,963,489 (Cho) discloses another reason for alignment. This patent shows a method for alignment of pattern-defining masks to each other, to enable pattern creation on a substrate between the two masks. Similarly, U.S. Pat. Nos. 4,835,078 and 4,904,087 (Harvey) show a method for alignment of two photomasks to each other prior to the insertion of a wafer between the two photomasks. The two photomasks are held aligned to each other and are not allowed to move, while the object or wafer to be processed can be freely inserted or removed between these two aligned photomasks without any consideration whether or not the object or wafer is aligned to the two photomasks.
The method of this subject invention not only teaches the alignment of the two artwork plates but also the alignment between the two artwork plates and a planar substrate. Furthermore, this invention discloses a means for verification of image pattern registration at a point when registration is most critical, namely, when the master image plates are in direct contact with the planar substrate immediately preceding the image generation process. This insures registration accuracy on each image transfer. This method may be utilized in any system requiring two side registration of pattern masters onto an opaque substrate.